Fault detection for laminated core

ABSTRACT

A method of evaluating the condition of a laminated core of an electric machine including positioning a magnetic flux injection excitation yoke extending between a pair of teeth of the laminated core, and the excitation yoke being wound with an excitation winding defining an electrical circuit for producing a magnetic flux excitation. Power is supplied to an excitation winding wound around the yoke to produce a magnetic flux in the yoke and to form a magnetic circuit through the yoke and the laminated core. A characteristic of the electrical circuit of the excitation winding is measured to identify a fault in the magnetic circuit corresponding to an eddy current between individual laminations in the laminated core.

FIELD OF THE INVENTION

The present invention relates to fault detection for laminated coresand, more particularly, to detecting and locating faults caused bydefects between laminations in a laminated core for an electric machine.

BACKGROUND OF THE INVENTION

Laminated stator cores, such as may be used in electric machines, may beformed by a plurality of laminations including a layer of insulatingmaterial located between adjacent laminations to prevent electricalconduction between the laminations. The laminated cores may be inspectedfor interlamination shorts during manufacture and during maintenanceoperations to identify conditions that may cause damage to the laminatedcore. The inspection operation may be performed using a measuring methodcomprising ring excitation of the stator lamination with a predeterminedinduction. This method, which indicates the effect of currents due tointerlamination shorts by local temperature differences, requires ahigh-power and high-voltage source and excitation windings with largecross sections.

In another inspection technique, the laminated core may be provided witha special winding to excite magnetic flux in the overall core, and a lowflux density is induced in the core. This inspection technique, known asan electromagnetic core imperfection detector (EL CID) test, provides aflux density in testing that is different than the flux density inoperation of the electric machine, and may not provide sufficient fluxto the laminated core teeth to provide a detection of insulation damagein the tooth area.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, a method of evaluatingthe condition of a laminated core of an electric machine is provided.The method comprises positioning a magnetic flux injection excitationyoke extending between a pair of teeth of the laminated core, theexcitation yoke having a pair of arms in engagement with the pair ofteeth, and the excitation yoke being wound with an excitation windingfor producing a magnetic flux excitation. Power is supplied to anelectrical circuit defined by the excitation winding to produce amagnetic flux in the excitation yoke and to form a magnetic circuitthrough the excitation yoke and the laminated core. A characteristic ofthe electrical circuit of the excitation winding is measured to identifya fault in the magnetic circuit corresponding to an eddy current betweenindividual laminations in the laminated core.

In accordance with another aspect of the invention, the measuring of thecharacteristic of the electrical circuit may include determining avariation in at least one of an impedance of the excitation winding anda current in the excitation winding corresponding to the fault in themagnetic circuit relative to a corresponding characteristic measured inthe excitation winding for a magnetic circuit created through theexcitation yoke and a portion of the laminated core without a fault.

In accordance with another aspect of the invention, the supplying powerto the excitation winding may comprise providing power to the excitationwinding at a constant voltage, and the measuring of the characteristicof the electrical circuit may include determining a variation in atleast one of an impedance of the excitation winding and a current in theexcitation winding corresponding to the fault in the magnetic circuitrelative to a corresponding characteristic measured in the excitationwinding for a magnetic circuit created through the excitation yoke and aportion of the laminated core without a fault.

In accordance with another aspect of the invention, the supplying powerto the excitation winding may comprise providing power to the excitationwinding at a constant current, and the measuring of the characteristicof the electrical circuit may include determining a variation in atleast one of an impedance of the excitation winding and a voltageapplied to the excitation winding corresponding to the fault in themagnetic circuit relative to a corresponding characteristic measured inthe excitation winding for a magnetic circuit created through theexcitation yoke and a portion of the laminated core without a fault.

In accordance with another aspect of the invention, the excitation yokemay include a second winding, and supplying power to the excitationwinding may comprise measuring a voltage in the second windingcorresponding to the magnetic flux in the excitation yoke and adjustinga voltage applied to the excitation winding with reference to thevoltage in the second winding to maintain a constant magnetic flux inthe excitation yoke.

In accordance with another aspect of the invention, in the case where aconstant magnetic flux is maintained in the excitation yoke, themeasuring of the characteristic of the electrical circuit may includedetermining a variation in at least one of an impedance of theexcitation winding and a current in the excitation winding correspondingto the fault in the magnetic circuit relative to a correspondingcharacteristic measured in the excitation winding for a magnetic circuitcreated through the excitation yoke and a portion of the laminated corewithout a fault.

In accordance with another aspect of the invention, a reference elementmay be provided having an output characteristic corresponding to themeasured characteristic of the excitation winding, and thecharacteristic of the excitation winding is compared to the outputcharacteristic of the reference element to identify the fault in themagnetic circuit.

In accordance with another aspect of the invention, the electricalcircuit of the excitation winding may comprise a bridge circuit in whichthe excitation winding and the reference element are each located in arespective branch of the bridge circuit.

In accordance with another aspect of the invention, the referenceelement may include a reference yoke wound with a reference winding, andthe reference winding may be connected to the bridge circuit.

In accordance with another aspect of the invention, the reference yokemay be located a predetermined distance from the excitation yoke and maybe moved along the laminated core with the excitation yoke to provide afault indication at different locations along the laminated core.

In accordance with another aspect of the invention, the reference yokemay be located at a fixed location and may span between a pair of teethof the laminated core at a location known to be free of defects betweenlaminations of the laminated core to include the laminated core in amagnetic circuit of the reference yoke.

In accordance with another aspect of the invention, the reference yokemay be located in a magnetic circuit at a location spaced from thelaminated core.

In accordance with another aspect of the invention, the referenceelement may include a capacitor.

In accordance with a further aspect of the invention, a method ofevaluating the condition of a laminated core of an electric machine isprovided. The method comprises positioning a magnetic flux injectionexcitation yoke extending between a pair of teeth of the laminated core,the excitation yoke having a pair of arms in engagement with the pair ofteeth, and the excitation yoke being wound with an excitation windingfor producing a magnetic flux excitation. A reference element isprovided having an impedance that is substantially the same as animpedance of the excitation, and a bridge circuit is provided in whichthe excitation winding and the reference element are each located in arespective branch of the bridge circuit. Power is supplied to theexcitation winding to produce a magnetic flux in the excitation yoke andto form a magnetic circuit through the excitation yoke and the laminatedcore, wherein one of a current and a voltage in the branches of thebridge circuit are balanced when the excitation yoke is positioned at alocation where there are no faults in the laminated core. An imbalanceis determined between the branches of the bridge circuit when at leastone of the excitation yoke and the reference element is located adjacentto a fault formed by a short between adjacent laminations in thelaminated core.

In accordance with a further aspect of the invention, the referenceelement may include a reference yoke wound with a reference winding, andthe reference winding may be connected to the bridge circuit.

In accordance with a further aspect of the invention, the reference yokemay include a pair of arms located in engagement with a pair of teeth ofthe laminated core to form a magnetic circuit through the reference yokeand the laminated core.

In accordance with a further aspect of the invention, the reference yokemay be located in a magnetic circuit separate from the laminated core.

In accordance with a further aspect of the invention, the bridge circuitmay comprise a Maxwell-Wein bridge circuit and the reference element mayinclude a capacitor providing a capacitive impedance balanced against aninductance of the excitation coil.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed that thepresent invention will be better understood from the followingdescription in conjunction with the accompanying Drawing Figures, inwhich like reference numerals identify like elements, and wherein:

FIG. 1A is a perspective view of an apparatus for evaluating thecondition of a laminated core in accordance with the present invention;

FIG. 1B is a perspective view illustrating a further aspect for theapparatus for evaluating the condition of a laminated core in accordancewith the present invention;

FIG. 2 is a schematic view illustrating an equivalent circuit for theapparatus illustrated in FIG. 1A;

FIG. 3 is a schematic view illustrating an alternative equivalentcircuit for the apparatus illustrated in FIG. 1A;

FIG. 4 is a schematic view illustrating an equivalent circuit for theapparatus illustrated in FIG. 1B;

FIG. 5 is a schematic view illustrating a bridge circuit for evaluatingthe condition of a laminated core with reference to a reference element;

FIGS. 6A-C are schematic views illustrating alternative arm elements foruse in a bridge circuit for evaluating the condition of a laminatedcore;

FIG. 7 is a schematic view illustrating an alternative bridge circuitfor evaluating the condition of a laminated core with reference to areference element;

FIGS. 8A-C are perspective views of alternative configurations for anapparatus including a reference element for evaluating the condition ofa laminated core in accordance with the present invention; and

FIG. 9 is a perspective view of an alternative apparatus for evaluatingthe condition of a laminated core.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration, and not by way oflimitation, specific preferred embodiments in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand that changes may be made without departing from the spirit and scopeof the present invention.

Referring to FIG. 1A, an apparatus 10 is illustrated for evaluating thecondition of the magnetic circuit of a laminated core 12 of an electricmachine, such as a laminated stator core in a generator. The laminatedcore 12 may comprise an assembly of superimposed laminations of magneticmaterial, each coated with an electrically insulating material to form,for example, a stator having a face 14 defined by a plurality of spacedapart radially projecting teeth 16, and associated slots 17 locatedbetween the teeth 16. As is described further below, the apparatus 10may be used to detect faults, such as may be caused by defects in theinsulating material between laminations, and resulting electrical shortsbetween adjacent laminations, as indicated by a variation in themagnetic flux conducted through the magnetic circuit passing through theteeth 16 relative to the magnetic flux conducted through a magneticcircuit formed by the teeth 16 at a different location, i.e., at alocation defined by a portion of the laminated core 12 that does nothave a fault.

The apparatus 10 includes an excitation yoke 18 comprising a U-shapedferromagnetic member, which may comprise a plurality of laminatedsheets, and including a pair of arms 20, 22 connected by a transverseportion 24. The transverse portion 24 of the excitation yoke 18 is woundwith a magnetic flux generating excitation winding 26. The excitationyoke 18 and winding 26 form a probe structure 27 for generating amagnetic flux in the laminated core 12. During a testing procedure forevaluating the condition of the laminated core 12, the yoke 18 ispositioned with the arms 20, 22 in close proximity to a pair of thelaminated core teeth 16, wherein ends of the arms 20, 22 are preferablylocated at a spacing matching the spacing of the pair of teeth 16 andany air gap between the arms 20, 22 and the teeth 16 is preferably keptto a minimum. An alternating voltage or current is provided to anelectrical excitation circuit 28 comprising the winding 26 to produce amagnetic flux in the yoke 18 and to inject a magnetic flux excitationinto the laminated core 12. The excitation circuit 28 may include apower source 30, i.e., a voltage or current source, which may provide analternating voltage or current at a frequency of 50 or 60 Hz, or mayoperate at a predetermined frequency from 60 Hz to 800 Hz. The magnetflux is generated in a local area of the laminated core 12 lengthlocated near the pair of teeth 16 spanned by the yoke 18. The magneticflux density generated by the yoke 18 is preferably the same as orcomparable to the magnetic flux density generated in the laminated core12 during operation of the electric machine, such as during use of thelaminated core 12 in an electrical generator.

The magnetic circuit of the yoke 18 is coupled with a magnetic circuit32 of a test zone of the laminated core 12, and the two coupled magneticcircuits form a coupled magnetic test circuit 33 (FIG. 2). Analternating current provided to the winding 26 is limited by animpedance of the winding 26 in association with the yoke 18 that hasactive and reactive components. Referring to FIG. 2, illustrating anequivalent circuit for the test zone magnetic circuit 32 and associatedexcitation circuit 28 of the coupled magnetic test circuit 33, thecurrent in the winding 26 has a flux linkage with eddy currents in thelaminated core 12 through the yoke 18, where the effects of eddycurrents in the individual laminations are represented by circuitportion 34 having an equivalent resistance 36 and leakage inductance 38.In addition, in the case where shorts or faults exist between adjacentlaminations, such as where insulation is missing between laminations, asdepicted at 31 in FIGS. 1A and 1B, the flux linkage with the winding 26may include circulating currents formed by the shorts in the laminatedcore 12, as illustrated by circuit portion 40. A short betweenlaminations is represented by the switch 42 being located in the closedposition (represented by dotted line) to include an equivalentresistance 44 and inductance 46 in the test zone magnetic circuit 32.The equivalent resistance 44 may represent or correspond to circulatingcurrents passing through the shorts between the laminations, and theequivalent inductance 46 may represent or correspond to leakageinductance of circulating currents passing through the shorts betweenthe laminations.

FIG. 3 illustrates an equivalent electrical circuit 33′ similar to anequivalent circuit of a transformer that includes a test zone portion32′ and associated excitation circuit portion 28′. The excitationcircuit portion 28′ includes a winding 26 represented by a resistance48, a mutual inductance 52, and a leakage inductance 50. The mutualinductance 52 is further included in the test zone portion 32′, whereinthe mutual inductance 52 represents the mutual inductance between thewinding 26 and 1) eddy currents in the laminations; and 2) circulatingcurrents through shorts between laminations. An inductance 56 representsleakage inductance of eddy currents within the individual laminations,and a resistor 54 represents an equivalent resistance of eddy currentswithin the individual laminations. In addition, in the case where shortsor faults exist between adjacent laminations in the test zone circuitportion 32′, the flux linkage with the winding 26 may includecirculating currents formed by the shorts in the laminated core 12, asillustrated by circuit portion 40′. A short between laminations isrepresented by the switch 42′ located in the closed position(represented by dotted line) to include an equivalent resistance 44′ andinductance 46′ in the test zone circuit 32′. The equivalent resistance44′ limits circulating currents passing through the shorts between thelaminations, and the equivalent inductance 46′ represents leakageinductance of circulating currents passing through the shorts betweenthe laminations.

As illustrated by the equivalent circuits of FIGS. 2 and 3, the currentin the winding 26 and its impedance at a fixed or predetermined voltagemay vary depending on the flux linkage of the winding 26 to itself (selfinductance), the value of eddy currents in individual laminations linkedwith the magnetic flux through the winding 26 (represented by 36 and 38in FIGS. 2; and 52, 54, 56 in FIG. 3), and circulating currents betweenlaminations linked with the magnetic flux through the winding 26(represented by 44, 46 in FIGS. 2; and 44′, 46′ in FIG. 3). If the yoke18 and associated winding 26 are moved longitudinally along thelaminated core 12, i.e., along the lengthwise direction of the teeth 16,a characteristic of the electrical circuit 28 of the excitation winding26 may vary.

Hence, different characteristics or parameters may be used or monitoredas an indicator of faults or shorts between the laminations. Forexample, in the case that the power supply 30 is operated withoutspecial stabilization, i.e., without additional components to stabilizeor compensate for small variations from a predetermined voltage orcurrent, a monitoring module 58 associated with the excitation circuit28 may monitor either the current in the winding 26 or the impedance ofthe winding 26 as an indicator of a lamination short. Similarly, in acase where the power source 30 is a constant voltage power source, i.e.,with stabilized voltage, the monitoring module 58 may also monitoreither the current in the winding 26 or the impedance of the winding 26as an indicator of a lamination short. In a case where the power source30 is a constant current power source, i.e., with stabilized current,the monitoring module 58 may monitor either a voltage applied to thewinding 26 or the impedance of the winding 26 as an indicator of alamination short.

In accordance with a further aspect of the apparatus 10, the powersource 30 may be controlled such that a constant flux is maintained inthe yoke 18. Referring to FIGS. 1B and 4, where FIG. 4 illustrates amodified form of the equivalent circuit of FIG. 3, the yoke 18 may beprovided with a second winding 60 connected to a regulator 62 forcontrolling a voltage output of the power supply 30. The magnetic fluxin the yoke 18 generates an AC voltage in the second winding 60 that isproportional to the magnetic flux in the yoke 18. The regulator 62monitors the output AC voltage of the second winding 60 to automaticallycontrol the output voltage of the power source 30 so as to maintain themagnetic flux in the yoke 18 substantially constant. In accordance withthis configuration of the apparatus 10, the monitoring module 58 maymonitor either a current in the winding 26 or the impedance of thewinding 26 as an indicator of a lamination short.

In each of the above described configurations for detecting a fault inthe laminated core 12, the monitored parameter (current, impedance orvoltage) may be compared to known values corresponding to a laminatedcore known to be without faults. The monitoring module 58 may provide adirect indication of the monitored parameter, such as by a meter orother read-out device. Alternatively, or in addition, the monitoringmodule 58 may digitize the results received as an indicator of a faultin the laminated core 12, such as by means of a microprocessor includedin the monitoring module 58, and store the digitized results in a memoryfor automated processing and analysis to identify fault locations in thelaminated core 12.

In accordance with a further aspect of the present apparatus 10, aconfiguration for identifying an indicator value corresponding to afault in the laminated core 12 is shown in FIG. 5. In particular, aconfiguration for determining a value received at the monitoring module58 relative to a reference value in a bridge circuit 64 is shown. Theconfiguration including the bridge circuit 64 may include a first upperarm defined by the portion of the excitation circuit 28 including theprobe structure 27 (excitation yoke 18 and winding 26), as describedwith reference to FIGS. 1A and 1B. The bridge circuit 64 may furtherinclude a second upper arm defined by a reference element 66 which maycomprise, for example, one of a reference winding, a referenceinductance or a reference capacitance, as is described further below.First and second lower arms 68, 70 of the bridge circuit 64 may compriseidentical elements including, for example, a resistor 72 and inductor 74(FIG. 7A), resistor(s) 76 (FIG. 7B), or a resistor 78 and capacitor 80(FIG. 7C). It should be noted that the resistor 72 and inductor 74 ofFIG. 7A may be connected either in series or in parallel, and theresistor 78 and capacitor 80 of FIG. 7C may be connected either inseries or parallel.

The first upper arm defined by the probe structure 27 and the firstlower arm 68 define a first branch 65 of the bridge circuit 64, and thesecond upper arm defined by the reference element 66 and the secondlower arm 70 define a second branch 67 of the bridge circuit 64. A meter82 may be provided, spanning between the two branches 65, 67 of thebridge circuit 64, to provide an indication of a current or voltagedifference between the two branches 65, 67. The meter 82 may comprise anoperator readable meter and/or may comprise circuitry of the monitoringmodule 58 to provide an analog or digital measurement indicative of avoltage difference or a current difference between the current orvoltage in the two branches 65, 67. In addition to the monitoring module58 including the voltage or current measuring component of the bridgecircuit 64 represented by the meter 82, the monitoring module 58 maycomprise one or more additional components of the bridge circuit 64 suchas, for example, the lower arms 68, 70 and/or the reference element 66of the bridge circuit 64.

FIG. 8A illustrates an exemplary configuration for the reference element66, where the reference element 66 comprises a reference yoke 84 woundwith a reference winding 86, and where the second upper arm of thebridge circuit 64 may comprise the reference winding 86 (FIG. 6) and thelaminated core 12 is included as part of the magnetic circuit of thereference element 66. The reference winding 86 may be connected to thepower supply 30, where the power supply 30 provides a power output tothe reference winding 86 comprising a substantially identical currentand/or voltage to that output by the power source 30 to the excitationwinding 26. The reference yoke 84 and reference winding 86 preferablycomprise elements that are substantially identical to the excitationyoke 18 and excitation winding 26, respectively, such that the referenceyoke 84 and reference winding 86 will exhibit a substantially identicalimpedance, i.e., substantially identical resistance and substantiallyidentical reactance, to the impedance of the excitation yoke 18 andexcitation winding 26 under identical conditions. That is, when both theprobe structure 27 and the reference element 66 are positioned atlocations along the laminated core 12 where there are no laminationshorts, the meter 82 of the bridge circuit 64 will detect a voltage orcurrent difference between the two bridge circuit branches 65, 67 thatis close to zero. If the probe structure 27 is located near a laminationshort 31 (FIG. 1A), such that a circulating flux is generated, theimpedance of the winding 26 will change, creating an imbalance in thebridge circuit 64. The imbalance in the bridge circuit 64 may bedetected by the meter 82 as an increase in voltage or current, providingan indication of the presence of the lamination short.

In accordance with one aspect of the reference element 66, as describedabove with reference to FIG. 8A, the probe structure 27 may define afirst probe, and the reference element 66 may define a second probeconnected or supported for movement with the probe structure 27 andspaced a predetermined distance, d, from the probe structure 27. Theconnected probe structure 27 and reference element 66 form a probeassembly 90. The power from the power sources 30 is connected to thewindings 26 and 86 such that the windings 26 and 86 create a magneticflux of the same polarity on the common or same teeth 16 of thelaminated core 12, assuming the windings 26 and 86 operate with the sameimpedance. In addition, the distance, d, between the probe structure 27and the reference structure 66 is preferably great enough to avoid asignificant flux linkage between the windings 26 and 86, in the eventthat an impedance difference were to occur in the windings 26 and 86.During an inspection operation, the probe assembly 90 may be moved alongthe longitudinal direction, L, of the laminated core 12. When either theprobe structure 27 or the reference element 66 encounters a short in thelaminations of the laminated core 12, the impedance of the respectivewinding 26, 86 adjacent to the lamination short will change relative tothe impedance of the other winding 26, 86, changing the balance in thebridge circuit 64, as may be indicated by a change in current or voltageon the meter 82.

In accordance with another aspect of the reference element 66, asdescribed above with reference to FIG. 5, the probe structure 27 andreference element 66 may be substantially identical structures, such asis described above with regard to the reference yoke 84 and referencewinding 86 (FIG. 8A). However, as illustrated in FIG. 8B, the referencestructure 66 may be positioned at a stationary location L₁ on thelaminated core 12 with the reference yoke 84 extending between a pair ofteeth 16, such that the laminated core 12 forms part of the magneticcircuit of the reference element 66. The stationary location L₁ isselected to be at a portion of the laminated core 12 that is known to befree of faults or shorts between laminations. Further, the constructionof the reference element 66 and its location L₁ on the laminated core 12is such that the reference element 66 will exhibit an impedance which issubstantially identical to that of the probe structure 27 when the probestructure 27 is positioned at a location along the laminated core 12where there are no lamination shorts. The probe structure 27 may bemoved along the laminated core 12 in the longitudinal direction, L, todetect shorts between the laminations.

When the probe structure 27 encounters a short 31 (FIG. 1A) in thelaminations of the laminated core 12, the impedance of the winding 26will change relative to the impedance of the reference winding 86,changing the balance in the bridge circuit 64, as may be detected at themonitoring module 58, i.e., as may be indicated by a change in currentor voltage on the meter 82.

In accordance with a further aspect of the reference element 66, asdescribed above with reference to FIG. 5, the reference element 66 maybe located in spaced relation from the laminated structure 12, such thatthe laminated structure 12 is not part of a magnetic circuit of thereference element 66. As illustrated in FIG. 8C, the reference element66 may comprise a reference yoke 84 and reference winding 86 similar tothat described above with reference to FIG. 8A and may be associatedwith a reference magnetic circuit 12 _(R), or may comprise a differentstructure including a reference winding 86 forming a magnetic circuitseparate from the laminated core 12, and may be energized by the powersource 30. The winding 86 of the reference structure 66 may beconstructed to have the same inductance and impedance, or may have onlythe same impedance, as the winding 26 of the probe structure 27. Inaccordance with this aspect, the probe structure 27 may be moved alongthe laminated core 12 in the longitudinal direction, L, to detect shortsbetween the laminations. When the probe structure 27 encounters a short31 (FIG. 1A) in the laminations of the laminated core 12, the impedanceof the winding 26 will change relative to the impedance of the referencewinding 86, changing the balance in the bridge circuit 64, as may bedetected at the monitoring module 58, i.e., as may be indicated by achange in current or voltage on the meter 82.

Referring to FIG. 7, an alternative bridge circuit 64′ incorporating analternative reference element 66′ is illustrated, and is configured as aMaxwell-Wein bridge circuit. In this configuration, as with the previousconfiguration, the reference element 66′ is separate from the laminatedcore 12. The first upper arm of the first branch 65′ of the bridgecircuit 64′ is formed by the probe structure 27. The reference element66′ may comprise a capacitor 92 and a resistor 94, or may comprise onlythe capacitor 92, and defines the second lower arm in the second branch67′ of the bridge circuit 66′. In the event that the reference element66′ comprises both the capacitor 92 and the resistor 94, the capacitor92 and resistor 94 may be configured either in series, as shown, or inparallel. The first lower arm 68′ and the second upper arm 70′ maycomprise components that are the same as, or similar to, those describedfor the arms 68, 70 with reference to FIGS. 6A, 6B, and 6C. Since thephase shifts associated with the inductance 50 in the winding 26 of theprobe structure 27 will be opposite from the phase shifts in thecapacitor 92, the inductive impedance of the probe structure 27 may bebalanced out by the capacitive impedance of the reference element 66′located in the opposite arm of the bridge circuit 64′. In accordancewith this aspect, the probe structure 27 may be moved along thelaminated core 12 in the longitudinal direction, L, to detect shortsbetween the laminations. When the probe structure 27 encounters a short31 (FIG. 1A) in the laminations of the laminated core 12, the impedanceof the winding 26 will change relative to the impedance of the referenceelement 66′, changing the balance in the bridge circuit 64′, as may beindicated by a change in current or voltage on a meter 82′ locatedbetween the two branches 65′, 67′ of the bridge circuit 64′.

Referring to FIG. 9, an alternative apparatus 110 for evaluating thecondition of the magnetic circuit of the laminated core 12 isillustrated. The apparatus 110 includes a first probe assembly 127comprising a first yoke 118 and a first winding 126 wound around theyoke 118 and defining an excitation winding. The first yoke 118 maycomprise a U-shaped ferromagnetic member, which may comprise a pluralityof laminated sheets, and includes a pair of arms 120,122 connected by atransverse portion 124. The transverse portion 124 of the yoke 118 maybe wound with the first winding 126. During a testing procedure forevaluating the condition of the laminated core 12, the first yoke 118may be positioned extending between a first pair of teeth 16 a, 16 bwith the pair of arms 120, 122 in engagement with the first pair ofteeth 16 a, 16 b.

The apparatus 110 further includes a second probe assembly 107comprising a second yoke 108 and a second winding 106 wound around thesecond yoke 108 and defining a sensing winding. The second yoke 108 maycomprise a U-shaped ferromagnetic member, smaller than the first yoke118, and which may comprise a plurality of laminated sheets. The secondyoke 108 includes a pair of arms 100,102 connected by a transverseportion 104. The transverse portion 104 of the second yoke 108 may bewound with the second winding 106. During a testing procedure forevaluating the condition of the laminated core 12, the second yoke 108may be positioned extending between a second pair of teeth 16 c, 16 dwith the pair of arms 100, 102 in engagement with the second pair ofteeth 16 c, 16 d. The second pair of teeth 16 c, 16 d are locatedintermediate or between the first pair of teeth 16 a, 16 b.

As illustrated in FIG. 9, the first yoke 118 may span across three slots17 a, 17 b, 17 c, including the second pair of teeth 16 c, 16 d, and thesecond yoke 108 may span across the single slot 17 b locatedintermediate or between the other two slots 17 a and 17 b.Alternatively, the first yoke 118 may span across more than three slots17, and the second yoke 108 may be located on intermediate teeth to spanacross slots 17 selected from the slots 17 spanned by the first yoke118. In general, the second yoke 108 will span at least two fewer slots17 than the number of slots 17 spanned by the first yoke 118.

The second yoke 108 may be connected or rigidly attached to the firstyoke 118, such that the first and second yokes 118, 108 may be moved inunison along the laminated core 12. The second yoke 108 may be alignedin the same plane with the first yoke 118, with the transverse portion104 of the second yoke 104 located directly below or inwardly from thetransverse portion 124 of the first yoke 118. Alternatively, the secondyoke 108 may be in close proximity to but slightly offset in thelongitudinal direction, L, relative to the first yoke 118, to the extentthat the second yoke 108 is still located within a region of themagnetic flux generated by the first probe 127, as is discussed furtherbelow.

The probe assembly 127 may be connected to an electrical circuit 128including a power source 130 to provide an alternating voltage orcurrent to the winding 126 to produce a magnetic flux in a local regionof the laminated core near the probe assembly 127. The power source 130may comprise a power source providing an output as described above forthe power source 30 in the apparatus 10 to generate a magnetic fluxdensity the same as or comparable to the magnetic flux density generatedin the laminated core 12 during operation of the electric machine.

The second probe assembly 107 may comprise a sensor probe, where avoltage may be produced in the second winding 106 as a result of thesecond yoke 108 being magnetically linked through the second pair ofteeth 16 c, 16 d to the magnetic flux generated in the laminated core 12through the first probe assembly 127. That is, the second probe assembly107 may sense a portion of the flux produced by the first probe assembly127 as a sensed flux leakage through the second pair of teeth 16 c, 16d. A variation in the voltage as the structure comprising the first andsecond probe assemblies 127, 107 is moved along the laminated core inthe longitudinal direction, L, or a particular voltage value relative toa predetermined threshold, measured from the second probe assembly 107may provide an indication of a fault, i.e., a short, in the laminationsof the laminated core 12. The second winding 106 may be located in asecond circuit 109 including a monitoring module 158 which may comprisean operator readable meter for monitoring voltage. Alternatively, or inaddition, the monitoring module 158 may digitize the results received asan indicator of a fault in the laminated core 12, such as by means of amicroprocessor included in the monitoring module 158, and store thedigitized results in a memory for automated processing and analysis toidentify fault locations in the laminated core 12.

In accordance with a further aspect of the configuration illustrated inFIG. 9, in the event that the testing procedure using the apparatus 110is performed without generator stator windings present in the slots 17,magnetic shunts 121 may be placed in slots 17 a, 17 c defined betweenthe first pair of teeth 16 a, 16 b and the second pair of teeth 16 c, 16d, respectively. The shunts 121 may be constructed of a laminatedmagnetic material, and may be positioned within the slots 17 a, 17 c toredirect a larger portion of the magnetic flux to the second probeassembly 107. It is believed that the shunts 121, and associatedredirection of magnetic flux, may increase the sensitivity of theapparatus 110 to sensing a lamination short in the second pair of teeth16 b, 16 c. That is, the shunts 121 may increase the amount of magneticflux directed into the second pair of teeth 16 a, 16 b, increasing thestrength of a signal associated with a fault in the laminations.Further, the shunts 121 may be moved to different radial positionswithin the slots 17 a, 17 b, i.e., to different positions relative tothe tips of the teeth 16 a, 16 b, to facilitate identification of theparticular radial location of a lamination short along the teeth 16 a,16 b, as exemplified by the diagrammatically depicted lamination short131.

It should be understood that, although the apparatus 110 is describedabove with reference to the first winding 126 comprising the excitationwinding, and the second winding 106 comprising the sensing winding, thelocation of the excitation and sensing windings may be switched, suchthat the second probe 107 operates to provide a magnetic flux to thelaminated core 12 and the first probe 127 operates to sense the magneticflux in the laminated core 12 in order identify the location of a faultin the laminations.

In further aspects of the apparatus 110, the power supply 130 may beoperated without special stabilization relative to the current, or maycomprise a constant voltage power source. Alternatively, the powersource 130 may be controlled such that a constant flux is maintained inthe first yoke 118, where the flux in the first yoke may be sensed by aflux sensing winding 160, see FIG. 9, connected to a voltage regulator162 for controlling a voltage output of the power supply 130. Themagnetic flux in the first yoke 118 generates an AC voltage in the fluxsensing winding 160 that is proportional to the magnetic flux in theyoke 118. The regulator 162 may monitor the output AC voltage of theflux sensing winding 160 to automatically control the output voltage ofthe power source 130 so as to maintain the magnetic flux in the yoke 118substantially constant.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. A method of evaluating the condition of a laminated core of anelectric machine, comprising: positioning a magnetic flux injectionexcitation yoke extending between a pair of teeth of the laminated core,the excitation yoke having a pair of arms in engagement with the pair ofteeth, and the excitation yoke being wound with an excitation windingfor producing a magnetic flux excitation; supplying power to anelectrical circuit defined by the excitation winding to produce amagnetic flux in the excitation yoke and to form a magnetic circuitthrough the excitation yoke and the laminated core; measuring acharacteristic of the electrical circuit of the excitation winding toidentify a fault in the magnetic circuit corresponding to an eddycurrent between individual laminations in the laminated core.
 2. Themethod of claim 1, wherein measuring the characteristic of theelectrical circuit includes determining a variation in at least one ofan impedance of the excitation winding and a current in the excitationwinding corresponding to the fault in the magnetic circuit relative to acorresponding characteristic measured in the excitation winding for amagnetic circuit created through the excitation yoke and a portion ofthe laminated core without a fault.
 3. The method of claim 1, whereinsupplying power to the excitation winding comprises providing power tothe excitation winding at a constant voltage.
 4. The method of claim 3,wherein measuring the characteristic of the electrical circuit includesdetermining a variation in at least one of an impedance of theexcitation winding and a current in the excitation winding correspondingto the fault in the magnetic circuit relative to a correspondingcharacteristic measured in the excitation winding for a magnetic circuitcreated through the excitation yoke and a portion of the laminated corewithout a fault.
 5. The method of claim 1, wherein supplying power tothe excitation winding comprises providing power to the excitationwinding at a constant current.
 6. The method of claim 5, whereinmeasuring the characteristic of the electrical circuit includesdetermining a variation in at least one of an impedance of theexcitation winding and a voltage applied to the excitation windingcorresponding to the fault in the magnetic circuit relative to acorresponding characteristic measured in the excitation winding for amagnetic circuit created through the excitation yoke and a portion ofthe laminated core without a fault.
 7. The method of claim 1, whereinthe excitation yoke includes a second winding, and supplying power tothe excitation winding comprises measuring a voltage in the secondwinding corresponding to the magnetic flux in the excitation yoke andadjusting a voltage applied to the excitation winding with reference tothe voltage in the second winding to maintain a constant magnetic fluxin the excitation yoke.
 8. The method of claim 7, wherein measuring thecharacteristic of the electrical circuit includes determining avariation in at least one of an impedance of the excitation winding anda current in the excitation winding corresponding to the fault in themagnetic circuit relative to a corresponding characteristic measured inthe excitation winding for a magnetic circuit created through theexcitation yoke and a portion of the laminated core without a fault. 9.The method of claim 1, including a reference element having an outputcharacteristic corresponding to the measured characteristic of theexcitation winding, and the characteristic of the excitation winding iscompared to the output characteristic of the reference element toidentify the fault in the magnetic circuit.
 10. The method of claim 9,wherein the electrical circuit of the excitation winding comprises abridge circuit in which the excitation winding and the reference elementare each located in a respective branch of the bridge circuit.
 11. Themethod of claim 10, wherein the reference element includes a referenceyoke wound with a reference winding, and the reference winding isconnected to the bridge circuit.
 12. The method of claim 11, wherein thereference yoke is located a predetermined distance from the excitationyoke and is moved along the laminated core with the excitation yoke toprovide a fault indication at different locations along the laminatedcore.
 13. The method of claim 11, wherein the reference yoke is locatedat a fixed location and spans between a pair of teeth of the laminatedcore at a location known to be free of defects between laminations ofthe laminated core to include the laminated core in a magnetic circuitof the reference yoke.
 14. The method of claim 11, wherein the referenceyoke is located in a magnetic circuit at a location spaced from thelaminated core.
 15. The method of claim 10, wherein the referenceelement includes a capacitor.
 16. A method of evaluating the conditionof a laminated core of an electric machine, comprising: positioning amagnetic flux injection excitation yoke extending between a pair ofteeth of the laminated core, the excitation yoke having a pair of armsin engagement with the pair of teeth, and the excitation yoke beingwound with an excitation winding for producing a magnetic fluxexcitation; providing a reference element having an impedance that issubstantially the same as an impedance of the excitation; providing abridge circuit in which the excitation winding and the reference elementare each located in a respective branch of the bridge circuit; supplyingpower to the excitation winding to produce a magnetic flux in theexcitation yoke and to form a magnetic circuit through the excitationyoke and the laminated core, wherein one of a current and a voltage inthe branches of the bridge circuit are balanced when the excitation yokeis positioned at a location where there are no faults in the laminatedcore; and determining an imbalance between the branches of the bridgecircuit when at least one of the excitation yoke and the referenceelement is located adjacent to a fault formed by a short betweenadjacent laminations in the laminated core.
 17. The method of claim 16,wherein the reference element includes a reference yoke wound with areference winding, and the reference winding is connected to the bridgecircuit.
 18. The method of claim 17, wherein the reference yoke includesa pair of arms located in engagement with a pair of teeth of thelaminated core to form a magnetic circuit through the reference yoke andthe laminated core.
 19. The method of claim 17, wherein the referenceyoke is located in a magnetic circuit separate from the laminated core.20. The method of claim 10, wherein the bridge circuit comprises aMaxwell-Wein bridge circuit and the reference element includes acapacitor providing a capacitive impedance balanced against aninductance of the excitation coil.